Having Meghan at the event was a break in royal protocol for the Queen, but it's not the first time she has made an exception for the former Suits star to join them prior to her nuptials to Harry. Queen Elizabeth serves as the head of the Commonwealth-53 nations in Europe, Africa, Asia, and the Americas-and Commonwealth Day celebrates this.
The training is said to be standard for senior members of the royal family, with Harry's mum Diana reportedly going through something similar in 1983.
Cheryl's partner Liam Payne performed at the service, while Dr Andrew Bastawrous, an eye surgeon who has helped combat blindness in developing counties, gave a reflection.
William faced criticism a year ago when he missed the service to attend his controversial lads-only skiing holiday in Verbier.
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It will be just as important to Will and Harry's father, Prince Charles the Prince of Wales, when he succeeds her as monarch and as head of the Commonwealth: he has already taken over significant aspects of her role since the 91-year-old queen began cutting back on global travel some years ago.
Harry and Meghan signalled their commitment to the Commonwealth and its people when they both mentioned the institution - which has the Queen as its head - during an interview to mark their engagement.
The Commonwealth Service is the largest inter-faith gathering in the United Kingdom and is now being televised on the BBC.
The event celebrates the Queen's commitment to the Commonwealth, which is made up of almost 2.4 billion people across 53 countries.
Others in attendance to the ceremony were the Duke of York, the Princess Royal, the Countess of Wessex, the Duchess of Gloucester and Princess Alexandra. Around 2,000 people sat in the congregation including Prime Minister Teresa May, senior politicians from across the United Kingdom and Commonwealth, and over 800 school children and young people.
Later on Monday, Prince Charles and Camilla and Prince William will be guests of the Commonwealth Secretary-General, Rt Hon Patricia Scotland QC, at the annual reception held at Marlborough House, the home of the Commonwealth Secretariat.